U.S. patent application number 10/658275 was filed with the patent office on 2005-03-10 for sorfc system with non-noble metal electrode compositions.
This patent application is currently assigned to Ion America Corporation. Invention is credited to Hickey, Darren, Russell, Ian.
Application Number | 20050053812 10/658275 |
Document ID | / |
Family ID | 34226751 |
Filed Date | 2005-03-10 |
United States Patent
Application |
20050053812 |
Kind Code |
A1 |
Hickey, Darren ; et
al. |
March 10, 2005 |
SORFC system with non-noble metal electrode compositions
Abstract
A solid oxide regenerative fuel cell includes a ceramic
electrolyte, a first electrode which is adapted to be positively
biased when the fuel cell operates in a fuel cell mode and in an
electrolysis mode, and a second electrode which is adapted to be
negatively biased when the fuel cell operates in the fuel cell mode
and in the electrolysis mode. The second electrode comprises less
than 1 mg/cm.sup.2 of noble metal.
Inventors: |
Hickey, Darren; (Palo Alto,
CA) ; Russell, Ian; (Sunnyvale, CA) |
Correspondence
Address: |
FOLEY AND LARDNER
SUITE 500
3000 K STREET NW
WASHINGTON
DC
20007
US
|
Assignee: |
Ion America Corporation
|
Family ID: |
34226751 |
Appl. No.: |
10/658275 |
Filed: |
September 10, 2003 |
Current U.S.
Class: |
429/411 ;
429/417; 429/495; 429/527 |
Current CPC
Class: |
H01M 8/126 20130101;
H01M 4/9066 20130101; Y02E 60/50 20130101; H01M 4/9016 20130101;
Y02P 70/50 20151101; H01M 8/1253 20130101; Y02P 70/56 20151101;
H01M 4/9033 20130101; H01M 4/8621 20130101; H01M 4/8885 20130101;
H01M 8/186 20130101; H01M 8/04164 20130101; H01M 8/04097 20130101;
Y02E 60/528 20130101; Y02E 60/525 20130101 |
Class at
Publication: |
429/021 ;
429/030; 429/040; 429/013 |
International
Class: |
H01M 008/18; H01M
004/90; H01M 008/12 |
Claims
What is claimed is:
1. A solid oxide regenerative fuel cell, comprising: a ceramic
electrolyte; a first electrode which is adapted to be positively
biased when the fuel cell operates in a fuel cell mode and in an
electrolysis mode; and a second electrode which is adapted to be
negatively biased when the fuel cell operates in the fuel cell mode
and in the electrolysis mode; wherein the second electrode
comprises less than 1 mg/cm.sup.2 of noble metal.
2. The fuel cell of claim 1, wherein the second electrode comprises
less than 20 weight percent of noble metal.
3. The fuel cell of claim 2, wherein the second electrode comprises
less than 0.1 mg/cm.sup.2 of noble metal and less than 1 weight
percent of noble metal.
4. The fuel cell of claim 3, wherein the second electrode comprises
no noble metal or an unavoidable trace impurity amount of noble
metal.
5. The fuel cell of claim 3, further comprising a first device
which is adapted to provide a sufficient reducing atmosphere to the
second electrode when the fuel cell operates in the electrolysis
mode to prevent the second electrode from oxidizing.
6. The fuel cell of claim 5, wherein the first device comprises a
hydrogen conduit operatively connected to at least one of a
hydrogen compressor and a hydrogen fuel storage vessel.
7. The fuel cell of claim 6, wherein the hydrogen conduit is
operatively connected to a fuel inlet of a fuel cell stack and a
fuel outlet of the fuel cell stack is operatively connected to a
water-hydrogen separator.
8. The fuel cell of claim 7, wherein the water-hydrogen separator
is operatively connected to the fuel inlet of the fuel cell stack
and is adapted to provide water to the second electrode when the
fuel cell operates in the electrolysis mode.
9. The fuel cell of claim 6, further comprising a valve which is
adapted to bleed a first sufficient amount of hydrogen from at
least one of the hydrogen compressor and the hydrogen fuel storage
vessel through the hydrogen conduit to the second electrode to
prevent the second electrode from oxidizing when the fuel cell
operates in the electrolysis mode and which is adapted to provide
hydrogen fuel from the hydrogen storage vessel through the hydrogen
conduit to the second electrode in a second amount greater than the
first amount when the fuel cell operates in the fuel cell mode.
10. The fuel cell of claim 6, wherein the first device comprises a
forming gas conduit operatively connected to a forming gas storage
vessel.
11. The fuel cell of claim 6, wherein the first device comprises a
carbon monoxide conduit operatively connected to a carbon monoxide
storage vessel.
12. The fuel cell of claim 1, further comprising a first means for
providing a sufficient reducing atmosphere to the second electrode
when the fuel cell operates in the electrolysis mode to prevent the
second electrode from oxidizing.
13. The fuel cell of claim 12, wherein the first means is a means
for providing hydrogen fuel to the second electrode when the fuel
cell operates in the fuel cell mode.
14. The fuel cell of claim 13, further comprising: a second means
for providing water to the second electrode when the fuel cell
operates in the electrolysis mode; a third means for removing
oxygen generated at the first electrode when the fuel cell operates
in the electrolysis mode; a fourth means for providing an oxidizer
to the first electrode when the fuel cell operates in the fuel cell
mode; and a fifth means for removing water from the second
electrode when the fuel cell operates in the fuel cell mode.
15. The fuel cell of claim 5, wherein the second electrode
comprises at least one of Ni, Cu, Fe or a combination thereof with
an ionic conducting phase.
16. The fuel cell of claim 15, wherein the second electrode
consists essentially of a Ni-YSZ cermet.
17. The fuel cell of claim 15, wherein: the second electrode
consists essentially of a Ni-doped ceria cermet; and the
electrolyte comprises a doped ceria portion in contact with the
second electrode and a YSZ portion in contact with the first
electrode.
18. The fuel cell of claim 15, wherein: the electrolyte comprises
YSZ, doped ceria or a combination thereof; and the first electrode
comprises at least one of LSM, LSCo, LCo, LSF, LSCoF, PSM or a
combination thereof with an ionic conducting phase.
19. A solid oxide regenerative fuel cell, comprising: a first
electrode which is adapted to be positively biased when the fuel
cell operates in a fuel cell mode and in an electrolysis mode; a
second electrode which is adapted to be negatively biased when the
fuel cell operates in the fuel cell mode and in the electrolysis
mode, wherein the second electrode comprises less than 1
mg/cm.sup.2 of noble metal; and a first means for conducting oxygen
ions from the first electrode to the second electrode when the fuel
cell operates in the fuel cell mode and for conducting oxygen ions
from the second electrode to the first electrode when the fuel cell
operates in the electrolysis mode.
20. The fuel cell of claim 19, wherein the second electrode
comprises less than 20 weight percent of noble metal.
21. The fuel cell of claim 20, wherein the second electrode
comprises less than 0.1 mg/cm.sup.2 of noble metal and less than 1
weight percent of noble metal.
22. The fuel cell of claim 21, wherein the second electrode
comprises no noble metal or an unavoidable trace impurity amount of
noble metal.
23. The fuel cell of claim 19, further comprising a second means
for providing a sufficient reducing atmosphere to the second
electrode when the fuel cell operates in the electrolysis mode to
prevent the second electrode from oxidizing.
24. The fuel cell of claim 23, wherein the second means is a means
for providing hydrogen fuel to the second electrode when the fuel
cell operates in the fuel cell mode.
25. The fuel cell of claim 24, further comprising: a third means
for providing water to the second electrode when the fuel cell
operates in the electrolysis mode; a fourth means for removing
oxygen generated at the first electrode when the fuel cell operates
in the electrolysis mode; a fifth means for providing an oxidizer
to the first electrode when the fuel cell operates in the fuel cell
mode; and a sixth means for removing water from the second
electrode when the fuel cell operates in the fuel cell mode.
26. The fuel cell of claim 19, wherein: the first electrode
comprises at least one of LSM, LSCo, LCo, LSF, LSCoF, PSM or a
combination thereof with an ionic conducting phase; and the second
electrode comprises at least one of Ni, Cu, Fe or a combination
thereof with an ionic conducting phase.
27. The fuel cell of claim 26, wherein: the first electrode
consists essentially of LSM; and the second electrode consists
essentially of a Ni-YSZ cermet.
28. A solid oxide regenerative fuel cell, comprising: a first
electrode which is adapted to be positively biased when the fuel
cell operates in a fuel cell mode and in an electrolysis mode; a
second electrode which is adapted to be negatively biased when the
fuel cell operates in the fuel cell mode and in the electrolysis
mode, wherein the second electrode comprises less than 1
mg/cm.sup.2 of noble metal; a first means for conducting oxygen
ions from the first electrode to the second electrode when the fuel
cell operates in the fuel cell mode and for conducting oxygen ions
from the second electrode to the first electrode when the fuel cell
operates in the electrolysis mode; and a second means for providing
a sufficient reducing atmosphere to the second electrode when the
fuel cell operates in the electrolysis mode to prevent the second
electrode from oxidizing.
29. The fuel cell of claim 28, wherein the second electrode
comprises less than 20 weight percent of noble metal.
30. The fuel cell of claim 29, wherein the second electrode
comprises less than 0.1 mg/cm.sup.2 of noble metal and less than 1
weight percent of noble metal.
31. The fuel cell of claim 30, wherein the second electrode
comprises no noble metal or an unavoidable trace impurity amount of
noble metal.
32. The fuel cell of claim 28, wherein the second means is a means
for providing hydrogen fuel to the second electrode when the fuel
cell operates in the fuel cell mode.
33. The fuel cell of claim 32, further comprising: a third means
for providing water to the second electrode when the fuel cell
operates in the electrolysis mode; a fourth means for removing
oxygen generated at the first electrode when the fuel cell operates
in the electrolysis mode; a fifth means for providing an oxidizer
to the first electrode when the fuel cell operates in the fuel cell
mode; and a sixth means for removing water from the second
electrode when the fuel cell operates in the fuel cell mode.
34. The fuel cell of claim 28, wherein: the first electrode
comprises at least one of LSM, LSCo, LCo, LSF, LSCoF, PSM or a
combination thereof with an ionic conducting phase; and the second
electrode comprises at least one of Ni, Cu, Fe or a combination
thereof with an ionic conducting phase.
35. The fuel cell of claim 34, wherein: the first electrode
consists essentially of LSM; and the second electrode consists
essentially of a Ni-YSZ cermet.
36. A method of operating a solid oxide regenerative fuel cell,
comprising: operating the solid oxide regenerative fuel cell in a
fuel cell mode by providing a fuel to a negative electrode and
providing an oxidizer to a positive electrode to generate
electricity and water vapor at the negative electrode; operating
the solid oxide regenerative fuel cell in an electrolysis mode by
providing electricity to the fuel cell and providing water vapor to
the negative electrode to generate fuel at the negative electrode
and oxygen at the positive electrode; and providing a sufficient
reducing atmosphere to the negative electrode when the solid oxide
regenerative fuel cell operates in the electrolysis mode to prevent
the negative electrode from oxidizing, wherein the negative
electrode comprises less than 1 mg/cm.sup.2 of noble metal.
37. The method of claim 36, wherein the fuel and the reducing
atmosphere comprise hydrogen.
38. The method of claim 37, wherein the water to hydrogen ratio at
the negative electrode during the electrolysis mode is 8 or
less.
39. The method of claim 36, wherein the reducing atmosphere
comprises forming gas.
40. The method of claim 36, wherein the reducing atmosphere
comprises carbon monoxide.
41. The method of claim 36, wherein the negative electrode
comprises less than 20 weight percent of noble metal.
42. The method of claim 41, wherein the negative electrode
comprises less than 0.1 mg/cm.sup.2 of noble metal and less than 1
weight percent of noble metal.
43. The method of claim 42, wherein the negative electrode
comprises no noble metal or an unavoidable trace impurity amount of
noble metal.
44. The method of claim 43, wherein: the positive electrode
comprises at least one of LSM, LSCo, LCo, LSF, LSCoF, PSM or a
combination thereof with an ionic conducting phase; and the
negative electrode comprises at least one of Ni, Cu, Fe or a
combination thereof with an ionic conducting phase.
45. The method of claim 44, wherein: the positive electrode
consists essentially of LSM; and the negative electrode consists
essentially of a Ni-YSZ cermet.
46. The method of claim 36, wherein the reducing atmosphere does
not chemically participate in the electrolysis process and is
cycled through the fuel cell without being consumed.
47. The method of claim 46, wherein the fuel cell is cycled between
the fuel cell mode and the electrolysis mode at least 30 times.
48. The method of claim 47, further comprising: generating hydrogen
at the negative electrode in the electrolysis mode by electrolysis
of water vapor; providing remaining water vapor and the generated
hydrogen to a water-hydrogen separator to separate the hydrogen
from water; providing the separated hydrogen to a compressor;
providing a first portion of the compressed hydrogen to a hydrogen
storage vessel; and providing a second portion of the compressed
hydrogen to the negative electrode to maintain the sufficient
reducing atmosphere at the negative electrode.
Description
BACKGROUND OF THE INVENTION
[0001] The present invention is generally directed to fuel cells
and more specifically to reversible fuel cells and their
operation.
[0002] Fuel cells are electrochemical devices which can convert
energy stored in fuels to electrical energy with high efficiencies.
There are classes of fuel cells that also allow reversed operation,
such that oxidized fuel can be reduced back to unoxidized fuel
using electrical energy as an input.
[0003] One type of reversible or regenerative fuel cell is the
solid oxide regenerative fuel cell (SORFC) which generates
electrical energy and reactant product from fuel and oxidizer in a
fuel cell or discharge mode and which generates the fuel and
oxidant from the reactant product and the electrical energy in an
electrolysis or charge mode. The SORFC contains a ceramic
electrolyte, a positive or oxygen electrode and a negative or fuel
electrode. The electrolyte may be yttria stabilized zirconia
("YSZ") or doped ceria. The positive electrode is exposed to an
oxidizer, such as air, in the fuel cell mode and to a generated
oxidant, such as oxygen gas, in the electrolysis mode. The positive
electrode may be made of a ceramic material, such as lanthanum
strontium manganite ("LSM") having a formula (La,Sr)MnO.sub.3 or
lanthanum strontium cobaltite (LSCo) having a formula
(La,Sr)CoO.sub.3. The negative electrode is exposed to a fuel, such
as hydrogen gas, in a fuel cell mode and to water vapor (i.e.,
reactant product) in the electrolysis mode. Since the negative
electrode is exposed to water vapor, it is made entirely of a noble
metal or contains a large amount of noble metal which does not
oxidize when exposed to water vapor. For example, the negative
electrode may be made of platinum.
[0004] However, the noble metals are expensive and increase the
cost of the fuel cell. In contrast, the prior art acknowledges that
the negative electrodes cannot be made from a non-noble metal in a
SORFC because such electrodes are oxidized by the water vapor in
the electrolysis mode. For example, an article by K. Eguchi et al.
in Solid State Ionics 86-88 (1996) 1245-1249 states on page 1246
that a cell with Ni-YSZ electrodes is not suitable for a solid
oxide electrolyzer cell. The article further states on page 1247
that that a high concentration of steam (i.e., water vapor) caused
the deterioration of a Ni-YSZ electrode and that a noble or
precious metal negative electrode is preferred.
BRIEF SUMMARY OF THE INVENTION
[0005] One preferred aspect of the present invention provides a
solid oxide regenerative fuel cell, comprising a ceramic
electrolyte, a first electrode which is adapted to be positively
biased when the fuel cell operates in a fuel cell mode and in an
electrolysis mode, and a second electrode which is adapted to be
negatively biased when the fuel cell operates in the fuel cell mode
and in the electrolysis mode. The second electrode comprises less
than 1 mg/cm.sup.2 of noble metal.
[0006] Another preferred aspect of the present invention provides a
method of operating a solid oxide regenerative fuel cell,
comprising operating the solid oxide regenerative fuel cell in a
fuel cell mode by providing a fuel to a negative electrode and
providing an oxidizer to a positive electrode to generate
electricity and water vapor at the negative electrode. The method
further comprises operating the solid oxide regenerative fuel cell
in an electrolysis mode by providing electricity to the fuel cell
and providing water vapor to the negative electrode to generate
fuel at the negative electrode and oxygen at the positive
electrode. The method further comprises providing a sufficient
reducing atmosphere to the negative electrode when the solid oxide
regenerative fuel cell operates in the electrolysis mode to prevent
the negative electrode from oxidizing. The negative electrode
comprises less than 1 mg/cm.sup.2 of noble metal.
BRIEF DESCRIPTION OF THE DRAWINGS
[0007] FIG. 1 is a schematic illustration of a SORFC system
operating in an electrolysis mode according to a preferred
embodiment of the present invention.
[0008] FIG. 2 is a schematic illustration of a SORFC system
operating in a fuel cell mode according to a preferred embodiment
of the present invention.
[0009] FIG. 3 is a schematic cross section of a single SORFC
operating in the electrolysis mode according to a preferred
embodiment of the present invention.
[0010] FIG. 4 is a schematic cross section of a single SORFC
operating in the fuel cell mode according to a preferred embodiment
of the present invention.
[0011] FIG. 5 is a plot of current potential and power density
versus current density of a SORFC cell according to a specific
example of the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
[0012] The present inventors have realized that SORFC negative
(i.e., fuel) electrode may contain no noble metals or a small
amount of noble metals, such as less than 1 mg/cm.sup.2 of noble
metal, if a sufficient reducing atmosphere is provided to the
negative electrode when the fuel cell operates in the electrolysis
mode to prevent the negative electrode from oxidizing. The use of
cheaper and/or more common conductive materials in the negative
electrode reduces the cost of the SORFC and improves operational
performance.
[0013] As used herein, the term noble metal includes gold, iridium,
palladium, platinum, rhodium, osmium and silver. These metals are
also known as precious metals. Preferably, the negative electrode
contains less than 20 weight percent of noble metal. More
preferably, the negative electrode contains less than 0.1
mg/cm.sup.2 of noble metal and less than 1 weight percent of noble
metal. Most preferably, the negative electrode contains no noble
metal or an unavoidable trace impurity amount of noble metal.
Furthermore, it is preferred that the positive electrode also
contains no noble metal or an unavoidable trace impurity amount of
noble metal.
[0014] As used herein, the term SORFC (i.e., solid oxide
regenerative fuel cell) includes a ceramic electrolyte, a positive
or oxygen electrode which is adapted to be positively biased when
the fuel cell operates in a fuel cell mode and in an electrolysis
mode, and a negative or fuel electrode which is adapted to be
negatively biased when the fuel cell operates in the fuel cell mode
and in the electrolysis mode. Oxygen ions are conducted through the
ceramic electrolyte from the positive electrode to the negative
electrode when the fuel cell operates in the fuel cell mode and
from the negative electrode to the positive electrode when the fuel
cell operates in the electrolysis mode.
[0015] Any suitable materials may be used for the electrolyte and
the electrodes. For example, the negative electrode may comprise a
non-noble metal, such as at least one of Ni, Cu, Fe or a
combination thereof with an ionic conducting phase (i.e., a
cermet). In one preferred aspect of the invention, the negative
electrode consists essentially of a Ni-YSZ cermet (i.e., a
nickel-yttria stabilized zirconia cermet). Any suitable weight
ratio of nickel to YSZ may be used in the electrode, such as a
ratio of 30:70 to 95:5, preferably 65:35. The electrolyte may
comprise any suitable ceramic, such as YSZ and/or doped ceria.
[0016] In another preferred aspect of the invention, the negative
electrode consists essentially of a Ni-doped ceria cermet. Any
suitable weight ratio of nickel to doped ceria may be used in the
electrode, such as a ratio of 30:70 to 95:5, preferably 65:35. In
this case, the electrolyte preferably comprises a doped ceria
electrolyte or a combination electrolyte having a doped ceria
portion or layer in contact with the negative electrode and a YSZ
portion in contact with the positive electrode. The ceria may be
doped with any suitable dopant, such as a Sc dopant or a rare earth
dopant selected from Gd and Sm, in an amount sufficient to render
the ceria to be ionically conducting.
[0017] The positive electrode may comprise any suitable material.
Preferably, the positive electrode comprises a conductive
perovskite ceramic material selected from LSM, LSCo, LCo, LSF,
LSCoF, PSM or a combination thereof with an ionic conducting phase.
Lanthanum strontium manganite ("LSM") preferably has a formula
(La.sub.x,Sr.sub.1-x) MnO.sub.3 where x ranges from 0.6 to 0.99,
preferably from 0.8 to 0.85. Lanthanum strontium cobaltite ("LSCo")
preferably has a formula (La.sub.x,Sr.sub.1-x)CoO.sub.3 where x
ranges from 0.6 to 0.99, preferably 0.8 to 0.85. If x is equal to
one, then the electrode material comprises LCo. Lanthanum strontium
ferrite ("LSF") preferably has a formula (La.sub.x
Sr.sub.1-x)FeO.sub.3 where x ranges from 0.4 to 0.99, preferably
from 0.6 to 0.7. Lanthanum strontium cobalt ferrite ("LSCoF")
preferably has a formula
(La.sub.x,Sr.sub.1-x)(Fe.sub.y,Co.sub.1-y)O.sub.- 3 where x ranges
from 0.4 to 0.99, preferably from 0.6 to 0.7 and y ranges from 0.01
to 0.99, preferably from 0.7 to 0.8. Praseodymium Strontium
Manganite ("PSM") preferably has a formula
(Pr.sub.x,Sr.sub.1-x)MnO.sub.3 where x ranges from 0.6 to 0.99,
preferably from 0.8 to 0.85. The perovskite electrode materials may
optionally be admixed with the electrode ceramics, such as YSZ and
doped ceria, such as GDC (gadolinium doped ceria). Other suitable
pervoskite electrode materials may also be used.
[0018] As used herein, "a sufficient reducing atmosphere to prevent
the negative electrode from oxidizing" comprises any suitable
reducing gas which when mixed with water vapor provided to the
negative electrode during electrolysis mode prevents the negative
electrode from oxidizing to an extent which prevents it from
operating according to its designed parameters during its expected
life span, such as for at least one month, preferably at least one
year, such as one to ten years, for example. Preferably, hydrogen
is used as the reducing gas. However, other gases, such as forming
gas (a nitrogen/hydrogen mixture) and carbon monoxide may also be
used alone or in combination with hydrogen. The maximum ratio of
water vapor to reducing gas provided to the negative electrode
during the electrolysis mode depends on the material of the
negative electrode and on the type of reducing gas used. Some
negative electrode materials require more reducing gas to prevent
oxidation that other negative electrode materials. For example, if
a hydrogen reducing gas is used for a Ni-YSZ electrode, then the
water to hydrogen ratio is preferably 8 or less, for example 0.1 to
8, such as 0.4 to 5 or 0.44 to 1. However, the water to hydrogen
ratio may be different than the ratio provided above depending on
various factors, such as the electrode composition, the overall gas
composition provided to the negative electrode and other factors,
while still preventing the negative electrode from oxidizing to an
extent which prevents it from operating according to its designed
parameters during its expected life span. Preferably, the reducing
atmosphere (i.e., the reducing gas) does not chemically participate
in the electrolysis process and is cycled through the fuel cell
without being consumed.
[0019] FIG. 1 illustrates a SORFC system 1 operating in the
electrolysis or charge mode. The system 1 contains a schematically
illustrated SORFC 10. While only a single SORFC 10 is shown, it
should be understood that the system 1 preferably contains a stack
of SORFCs, containing a plurality of electrolytes, positive
electrodes and negative electrodes. The system 1 also contains a
fuel storage vessel 101, such as a hydrogen tank, an optional fuel
compressor 103, a water-hydrogen separator/water storage device
105, a water pump 107, an oxidizer blower 109, a fuel bleed valve
111 and optional water, oxidizer and compressor valves 113, 115 and
117, respectively. The system 1 also contains heat exchangers 119
and 121 which preheat the inlet streams into the fuel cell 10 using
the fuel cell exhaust streams. The system further contains fuel and
oxidizer conduits, such as pipes, hoses or other suitable gas and
liquid conduits, which connect the above mentioned components
together.
[0020] The system contains a reducing gas conduit 123 which
provides a sufficient reducing atmosphere to the negative electrode
of the fuel cell 10 when the fuel cell operates in the electrolysis
mode to prevent the negative electrode from oxidizing. Preferably,
the reducing gas 123 conduit also comprises a fuel conduit which is
used to provide fuel to the negative electrode during the fuel cell
or discharge mode. Thus, the reducing gas in the electrolysis mode
preferably, but not necessarily, comprises the same gas as the fuel
which is used in the fuel cell mode. In the electrolysis mode, the
bleed valve 111 located in the reducing gas conduit is partially
opened to provide a smaller amount of fuel/reducing gas to the fuel
cell than in the fuel cell mode.
[0021] Preferably, the reducing gas/fuel comprises hydrogen and the
reducing gas conduit 123 comprises a hydrogen conduit operatively
connected to at least one of a hydrogen compressor 103 and the
hydrogen fuel storage vessel 101. The term operatively connected
means that the conduit 123 may be directly or indirectly connected
to the compressor 103 and/or vessel 101 to allow hydrogen to flow
from the compressor 103 and/or vessel 101 through the conduit 123
into the fuel cell. The conduit 123 is operatively connected to the
fuel inlet of a fuel cell 10 (i.e., to the inlet of the fuel cell
stack).
[0022] The water-hydrogen separator 105 is also operatively
connected to the fuel inlet of the fuel cell via the water inlet
conduit 125. The separator 105 provides water to the negative
electrode of the fuel cell 10 when the fuel cell 10 operates in the
electrolysis mode. Preferably, the conduits 123 and 125 converge at
the three way valve 113, and inlet conduit 127 provides the water
and reducing gas from valve 113 to the negative electrode of the
fuel cell 10.
[0023] A fuel outlet of the fuel cell 10 is operatively connected
to a water-hydrogen separator 105 via a fuel exhaust conduit 129.
Conduit 129 removes water from the negative electrode when the fuel
cell operates in the fuel cell mode. An oxygen exhaust conduit 131
removes oxygen generated at the positive electrode when the fuel
cell operates in the electrolysis mode. An oxidizer inlet conduit
133 provides an oxidizer, such as air or oxygen, to the positive
electrode of the fuel cell 10 when the fuel cell operates in the
fuel cell mode. In the electrolysis mode, the conduit 133 is closed
by valve 115.
[0024] FIG. 2 illustrates the SORFC system 1 operating in the fuel
cell or discharge mode. The system 1 is the same, except that the
bleed valve 111 is opened to a greater amount than in the
electrolysis mode, the oxidizer valve 115 is open instead of closed
and the water valve 113 either totally or partially closes the
water conduit 125.
[0025] A method of operating the solid oxide regenerative fuel cell
system 1 will now be described. In the fuel cell mode shown in FIG.
2, a fuel, such as hydrogen, carbon monoxide and/or a hydrocarbon
gas, such as methane, is provided to the negative electrode of the
fuel cell 10 from storage vessel 101 through conduits 123 and 127.
The fuel is preheated in the heat exchanger 119. If desired, some
water from the separator/storage device 105 is provided via
conduits 125 and 127 to the negative electrode of the fuel cell as
well. Alternatively, the water may be provided from a water pipe
rather than from storage.
[0026] An oxidizer, such as oxygen or air is provided to the
positive electrode of the fuel cell 10 through conduit 133. This
generates electricity (i.e., electrical energy) and water vapor at
the negative electrode. The unused oxidizer is discharged through
conduit 131. The water vapor reactant product along with unused
fuel, such as hydrogen, and other gases, such as carbon monoxide,
are discharged from the fuel cell through conduit 129 into the
separator 105. The hydrogen is separated from water in the
separator and is provided into the compressor 103 through conduit
135. The compressor 103 cycles the hydrogen back into the fuel cell
10.
[0027] In the electrolysis mode shown in FIG. 1, electricity is
provided to the fuel cell. Water vapor is provided to the negative
electrode of the fuel cell 10 from the separator/storage device 105
or from a water pipe through conduits 125 and 127. A sufficient
reducing atmosphere, such as hydrogen gas, is also provided to the
negative electrode through conduits 123 and 127. For example, at
start-up of the SORFC operation, when the compressor 103 does not
usually run, the hydrogen may be provided from the storage vessel
101. Subsequently, when the compressor 103 becomes operational at
steady state, it provides hydrogen to the conduit 123 and to the
storage vessel 101.
[0028] This generates fuel, such as hydrogen, at the negative
electrode, and oxygen at the positive electrode of the fuel cell.
The hydrogen, including the hydrogen generated in the electrolysis
of water vapor reaction and the hydrogen provided from conduit 123
along with remaining unreacted water vapor are provided from the
fuel cell 10 through conduit 129 to the separator 105. The
water-hydrogen separator 105 separates the hydrogen from water,
with the water being either stored or discharged. The separated
hydrogen is provided to the compressor 103 through conduit 135. The
compressor provides a first portion of the compressed hydrogen to
the hydrogen storage vessel 101 and provides a second portion of
the compressed hydrogen to the negative electrode of the fuel cell
10 through conduit 125 to maintain the sufficient reducing
atmosphere at the negative electrode. The oxygen generated during
the electrolysis reaction is discharged through conduit 131.
[0029] Preferably, the fuel cell 10 is cycled between the fuel cell
mode and the electrolysis mode at least 30 times, such as 30 to
3,000 times. During the cycles, when the fuel cell operates in the
electrolysis mode, the bleed valve 111 bleeds a first sufficient
amount of hydrogen from at least one of the hydrogen compressor and
the hydrogen fuel storage vessel through the hydrogen conduit 123
to the negative electrode of the fuel cell to prevent the negative
electrode from oxidizing. Providing a reducing atmosphere on the
negative electrode during the electrolysis mode allows the use of
non-noble materials in the electrode which also maintains
compatibility for the electrolysis operation.
[0030] When the fuel cell operates in the fuel cell mode, the bleed
valve provides hydrogen fuel from the hydrogen storage vessel
through the hydrogen conduit 123 to the negative electrode in a
second amount greater than the first amount. In other words, the
first amount of reducing gas should be a small amount of reducing
gas, but sufficient to prevent oxidation of the negative
electrode.
[0031] It should be noted that the hydrogen conduit 123 provides a
sufficient amount of reducing gas to the plurality of negative
electrodes of a fuel cell stack to prevent all negative electrodes
of the stack from oxidizing. Therefore the negative electrodes of
the SORFC stack are maintained in a reducing atmosphere, preventing
oxidation of the electrode materials at elevated temperatures in
the range 600-1000.degree. C.
[0032] In alternative embodiments of the present invention,
separate storage vessels are used to store fuel and the reducing
gas. Preferably, this occurs when the fuel and reducing gas
comprise different gases. For example, the fuel may comprise a
hydrocarbon fuel rather than hydrogen, or forming gas or carbon
monoxide is used as a reducing gas. In this case, a separate
reducing gas storage vessel, such as a hydrogen, carbon monoxide or
forming gas storage tank or pipe may be used to provide the
reducing gas into the fuel cell 10 in the electrolysis mode, while
the fuel storage vessel 101 is used to provide fuel into the fuel
cell in the fuel cell mode.
[0033] A single SORFC 10 operating in the electrolysis mode is
shown in FIG. 3. The SORFC contains an anode (positive) electrode
11, an electrolyte 13 and a cathode (negative) electrode 12. An
anode gas chamber 14 is formed between the electrolyte 13 and an
anode side interconnect (not shown for simplicity). A cathode gas
chamber 15 is formed between the electrolyte 13 and a cathode side
interconnect (also not shown for simplicity).
[0034] A reaction product gas mixture 17 may contain primarily
water with reducing gas, such as hydrogen. Alternatively, the
reaction product gas mixture 17 may contain primarily water vapor
and carbon dioxide if a carbon containing gas or liquid, such as
methane, is used as a fuel. Hydrogen, carbon monoxide or forming
gas is also added to the gas mixture as the reducing gas.
[0035] The reaction product gas mixture 17 is introduced into the
cathode gas chamber 15. A direct current power source (not shown)
is connected to the anode electrode 11 and the cathode electrode 12
in such a way that when electrical current is flowing, the anode
electrode 11 takes on a positive voltage charge and the cathode
electrode 12 takes on a negative voltage charge. When the electric
current is flowing, the gas mixture 17 gives up oxygen ions 16 to
form cathode discharge mixture 19 consisting primarily of hydrogen
and optionally carbon monoxide if mixture 17 contained carbon
dioxide. Oxygen ions 16 transport across the electrolyte 13 under
the electrical current. The oxygen ions 16 are converted into the
oxidant, such as oxygen gas 18 on the anode electrode 11 under the
influence of the electrical current. The oxygen gas 18 is
discharged from the anode chamber 14, while the electrolysis
product (e.g., hydrogen and optionally carbon monoxide) is
collected from the cathode chamber. If carbon monoxide is present
in the product, then the product may be converted to methane fuel
and water in a Sabatier reactor.
[0036] A single SORFC 20 operating in the fuel cell mode is shown
in FIG. 4. SORFC 20 is the same as SORFC 10, except that the
cathode and anode designations of its electrodes are reversed.
Cathode (positive) electrode 21 is the same electrode as that
identified as the anode (positive) electrode 11 in FIG. 3 when
operating in the electrolysis mode. Anode (negative) electrode 22
is the same electrode as that identified as the cathode (negative)
electrode 12 in FIG. 3 when operating in the electrolysis mode.
Solid oxide electrolyte 23 is the same electrolyte as that
identified as electrolyte 13 in FIG. 4 when operating in the
electrolysis mode. Cathode gas chamber 24 is the same gas chamber
as that identified as the anode gas chamber 14 in FIG. 3 when
operating in the electrolysis mode. Anode gas chamber 25 is the
same gas chamber as that identified as the cathode gas chamber 15
in FIG. 3 when operating in the electrolysis mode.
[0037] A fuel gas 27 is introduced into the anode gas chamber 25.
An oxidizer, such as air or oxygen gas 28 is introduced into the
cathode chamber 24. The fuel may comprise hydrogen, a hydrocarbon
gas, such as methane, and/or carbon monoxide. Water may be added to
the fuel if desired. An electrical fuel cell load (not shown) is
applied to the SORFC 20 and the oxygen gas 28 forms oxygen ions 26
under the influence of the electrical load. Oxygen ions 26
transport across the electrolyte 23 under the influence of the
electrical current. On the anode electrode 22, the oxygen ions 26
combine with hydrogen and optionally carbon, if present, from gas
mixture 27 to form gas mixture 29 containing water vapor and
optionally carbon dioxide, if a carbon containing gas is present in
the fuel 27. Gas mixture 29 is discharged from the anode chamber
and stored as the reaction product. In the process described above,
the SORFC 20 has made electrical energy or power, which is output
through its electrodes.
[0038] The SORFC systems described herein may have other
embodiments and configurations, as desired. Other components, such
as fuel side exhaust stream condensers, heat exchangers,
heat-driven heat pumps, turbines, additional gas separation
devices, hydrogen separators which separate hydrogen from the fuel
exhaust and provide hydrogen for external use, fuel preprocessing
subsystems, fuel reformers, water-gas shift reactors, and Sabatier
reactors which form methane from hydrogen and carbon monoxide, may
be added if desired, as described, for example, in U.S. application
Ser. No. 10/300,021, filed on Nov. 20, 2002, in U.S. Provisional
Application Ser. No. 60/461,190, filed on Apr. 9, 2003, and in U.S.
application Ser. No. 10/446,704, filed on May 29, 2003 all
incorporated herein by reference in their entirety.
[0039] The following specific example is provided for illustration
only and should not be considered limiting on the scope of the
present invention. FIG. 5 illustrates the plot of cell potential
and power density versus current density for a single 10 cm.sup.2
SORFC cell using a test bed that models the inlet gas streams as
described with respect to FIGS. 1 and 2 above. The SORFC cell
contains the following components. The negative or fuel electrode
is a Ni-YSZ cermet electrode containing 65 weight percent Ni and 35
weight percent YSZ. This electrode is 27 microns thick and is made
by screen printing on the electrolyte and being fired to
1350.degree. C. The electrolyte is a YSZ electrolyte that is 300
microns thick. The electrolyte is tape cast and fired to
1550.degree. C. The positive or oxygen electrode is an LSM
electrode that is 39 microns thick. This electrode is made by
screen printing on the electrolyte and firing to 1200.degree.
C.
[0040] The negative electrode is fed with a constant 300 sccm of
H.sub.2 passing through a humidifier at a set temperature. The
charge (i.e., electrolysis) mode is run with the humidifier set to
70.degree. C. or 30.75% H.sub.2O. This provides an H.sub.2O to
H.sub.2 ratio of 0.44 to the negative electrode. One discharge
(i.e., fuel cell) mode is run with the humidifier set to 70.degree.
C. or 30.75% H.sub.2O, while another discharge (i.e., fuel cell)
mode is run with the humidifier set to 29.degree. C. or 3.95%
H.sub.2O. The H.sub.2O to H.sub.2 ratio is 0.44 and 0.04,
respectively, for the respective discharge mode runs. Table 1 below
lists the negative electrode conditions for the various modes of
operation with ambient pressure reactants.
1 HUMIDI- H.sub.2O H.sub.2 FLOW FIER TEMP PERCENT H.sub.2O/H.sub.2
OPERATING MODE [sccm] [.degree. C.] [%] RATIO Charge Mode 300 70
30.75 0.44 Discharge Mode 1 300 70 30.75 0.44 ("wet hydrogen fuel")
Discharge Mode 2 300 29 3.95 0.04 ("dry hydrogen fuel")
[0041] As shown in FIG. 5, this fuel cell with a negative electrode
which contains no noble metal is successfully operated in both
charge and discharge modes and exhibits acceptable current-voltage
and current-power characteristics for reversible operation.
[0042] The foregoing description of the invention has been
presented for purposes of illustration and description. It is not
intended to be exhaustive or to limit the invention to the precise
form disclosed, and modifications and variations are possible in
light of the above teachings or may be acquired from practice of
the invention. The description was chosen in order to explain the
principles of the invention and its practical application. It is
intended that the scope of the invention be defined by the claims
appended hereto, and their equivalents.
* * * * *